1. Field of the Invention
The present invention relates to a vehicle having two hydrostatic stepless transmissions (HSTs); one is a traveling HST for driving the vehicle either forward or backward at various speeds, and the other is a steering HST for turning the vehicle rightward or leftward at various relative speeds of right and left drive wheels.
2. Related Art
As well known from U.S. Pat. No. 6,257,357, there is a conventional hydraulically driven and steered vehicle having two HSTs, i.e., the traveling HST and the steering HST. The traveling HST is provided for driving left and right drive wheels forward or backward at the same various speed, and the steering HST is provided for differentially driving the left and right drive wheels so as to turn the vehicle leftward or rightward. To change the rotational speed and direction of respective hydraulic motors of the HSTs, the vehicle is provided with a speed control operation device, such as an accelerator pedal, operatively connected to the traveling HST, and with a steering operation device, such as a steering wheel, operatively connected to the steering HST.
In the hydraulically controlled vehicle having the traveling HST and steering HST, the most complex and important point for constructing a linkage between the two HSTs is to ensure a reverse steering logic mechanism, i.e., to make the right and left turning direction coincide with the rotational direction of the steering wheel whether the vehicle travels forward or backward. The disclosed conventional vehicle is provided with a purely mechanical reverse steering logic mechanism with no assist of a hydraulic mechanism, which is complicated in assembly. Further, while the HSTs are disposed between the rear axles, the steering control linkage (i.e., the reverse steering logic mechanism, including the sector gear meshing with the pinion on the bottom end of the steering wheel shaft), is extended in front of the bottom end of the steering wheel shaft so as to be connected to the accelerator pedal, thereby making a large dead space between the bottom end of the steering wheel shaft and the HSTs therebehind in the fore-and-aft direction, and thereby prevents reduction of the vehicle.
There is another conventional reverse logic mechanism, which is not provided in the steering control linkage but in the steering HST, as disclosed in JP 2003-312528A and US 2003/0201134A1. This is constituted by a directive control valve disposed across the pair of oil passages between the pump and motor of the steering HST so as to exchange the ports of motor to be connected to the suction and delivery ports of pump depending upon whether the vehicle travels forward or backward. When the vehicle travels forward and the steering wheel is turned from its straight traveling position, one oil passage is hydraulically pressured higher than the other. The turning vehicle sometimes requires its traveling direction to be reversed between forward and backward. However, even if the angle of the steering wheel is kept, the exchange of port-connection between the pump and motor in the steering HST by the reverse of traveling direction causes that the depressed oil passage suddenly becomes higher-pressured. Such fluctuation of oil flow and pressure in the closed oil circuit of the steering HST causes an operator to feel discomfort.